Methods of dispersing fogs



States Patent METHODS OF DISPERSING FOGS George Alfred Hugh Elton, London, England, assignor of one-half to Leda Chemicals Limited, London, England, a British company I No Drawing. Application April 30, 1954- Serial No. 426,924

chums priority, application Great Britain 'May 4, 1953 13 Claims. '(cl. 239-2 This invention relates to methods of dispersing fogs.

Most fogs consist of liquid droplets whose diameters vary from 1 to 100 microns and may be of the order of ten microns. These droplets are often formed by condensation of water vapour on a solid nucleus, such as a smoke particle. The droplets may also contain varying amounts 'ofelectrolytes in solution, depending on atmospheric conditions, and the surfaces of the droplets gen erally take up a charge. This charge varies with the type of ion adsorbed, because different types of ions are adsorbed to diiferent'extents (depending on the magnitudes of their adsorption energies). Droplets of pure water dispersed in air carry a negative surface charge of about 30. e.=s.u. per square cm., and although it is possible that 7 ways of introducing a surface-active agent into a fog now be described in greater detail.

THE SMOKE METHOD The smoke may be formed in various ways, e.-g. a spray of a dilute solution of a surface-active agent in a volatile solvent may be introduced into astream of :hot air, which volatilises the solvent, and produces .a smoke.' Smoke particles captured by liquid fog droplets, dissolve, and cause a lowering of the droplet surface charge by adsorption. If an excess of ionic smoke is used, it is possible by this method to produce reversal of the charge, and hence -:re stabilisation of the fog. .It is therefore necessary when treating a fog 'to be ableto assess the magnitude ofthe surface charge. .For an uncontaminated water fog, :the charge is -30 e.s.u.xper cmfi. This can be removed by tetraisoamylammonium chloride :at a concentration of about 1-3 X l()-' N (see Gilman and Bach, Acta Physicochimica 'U.R.S.S. 1938, 9, 27). With a contaminated fog, the charge on the droplets maybe determined by collecting a sample of the vfog water and observing the other ions will also be adsorbed, the droplets in most fogs Willihave a negative charge unless some of the other ions have a large positive charge or are strongly adsorbed; When two such droplets in a fog approach close to onei another, they will repel oneanother, the extent of repulsionrdepeuding on the magnitude of the surface charges. This repulsion tends to prevent collision and coalescence.

Broadly according to the present invention there is provided 'a method of dispersing fog, wherein a surfaceactive agent is introduced in the fog in such manner and insuch proportionsas to promote the coalescence of the droplets of the fog and their precipitation as rain or drizzle.

This coalescence may be promoted by removing or lowering the surface charge on the droplets of the fog. As the droplets "grow by coalescence, their velocity of sedimentation under gravity increases, and eventually the liquid water -con't ent*of the fogis precipitated as rain or drizzle. The same effect can be produced by introducing into the fog oppositely charged droplets which, at close distances of approach, attract the fog droplets.

"""Ihe :surface charge maybe removed or lowered by introducing a surface-active agent, as a smoke, into the re droplets. The'surface active agent should be a noncfone' or,jpre'ferably, an ionic material having a charge opposite to that of the fog droplets. When coalescence i'sto be caused by attraction of fog droplets "to oppositely charged droplets "containing surface-active agent, the latter may be introduced into the fog in the form of a PITW- Negatively charged 'fogdrop'lets are the most common in occurrence and cationic surface-active agents such as tetraisoamylammonium chloride, lauryl or stearyl diin e'th'yl benizylainjrnonium chloride or cetyl pyridinium chloride may be used 't'o disperse a fog composed of such dropletsui If analysis of the fog droplets shows that already, under atmospheric conditions, they contain appreciable amounts of surface active or m'ultivalent cations, andhence have 'a positive surface charge precipitation can bebrought about-by surface active anions, e.g. salts of'long-chain alkyl sulphates or sulphonates.

"For a better understanding of the invention, the two rate of cataphoresis of an air bubble in the liquid. I'Experience with fogs of various types enables a reasonable estimate to be made of the probable magnitude of Zthe surface charge. The amounts of surface activie ag'e'nt necessary to remove this charge can then be determined from a calibration chart; e.'g. the calculated amounts :of tetraisoamylammon'ium chloride necessary to. neutralise the charge of various types of 'fog droplets are :given :in the table.

Table i I contain-- 1 Type'offog M0un-; Seafog -natedtain fog industrial Charge,e.s.ut/em. eat-50' -'-100't0 -2o0 -ca.-1,no0 Normality 0f catiOn-actiV agentrequired 5 l0- 5x101, 10

Once it has-been decided what concentration of ionic surface-active agent is necessary, an appropriate amount of smoke is dispersed into the fog, depending on the amount of liquid w'ate'r'p'er unit volume of the fog, which can be determined by standard 'methods. For example, if it is required to treat a contaminated fog of surface charge l'000 e.s.u./cm. and the liquid water content of the air is 3 10"' gm./cc., .then 1 10 cubic metres (i.e. 1 "10 cc.) contains 3x10 of water '(30 litres). In order to bring the concentration to 10* N it is required tohave 30x10 gm. equivalents of laurylidimethyl benzylamnionium chloride (equivalent weight 340), that --is about 1 gm. is required in 1X10 cubic metres. If-the height to which the fog is dispersed is metres, this requires the release of 1 gm. of smoke per thousand square metres or 1 kg. per square kilometre.

:Non-ionic surface-active agents (e.g. pentaeryth'ritol esters, polyoxyethylene compounds) can lower the surface charge, not by a neutralization process .(as is the case with the ionic agents), but by displacement of ions from the surface. That is, if sufiicient non-ionic agent is added to a fog droplet to produce a complete, or nearly complete, absorbed film of the agent in the sur-. face, the charge-forming ions are displaced. Fairly large amounts of such compounds are required for vthis (:s'ay ca 10* N to be certain, but non-ionic compounds have the advantage for use in the smoke method that fov'erdosing is not possible). For dealing with light fogs or for use in conditions where the composition and water content of the fog is unknown or subject to sudden change,the use of non-ionic compounds maybe the most satisfactory;

THE SPRAY METHOD An ionic surface-active agent can be used in a spray of water droplets which are introduced in the fog by means of a, machine of ,high spraying rate (eg. of

the type used to spray insecticides on a;large scale). In this method sufiicient surfaceaactive agent is added to the sprayed liquid to give the drops an opposite charge to those in the fog, and hence collision is promoted by attraction. Furthermore, fairly large 'drops. can be sprayed, if necessary many times larger thangthe fog droplets. This has the advantage that the hydrodynamic collision eiiiciency is larger for large droplets than for small droplets (Langmuir, J. Meteorology, 1948, 5, 175). That is, even if there are no charges on the dropets, not all of the expected collisions actuallyoccur, as interaction of the hydrodynamic flow lines associated with the droplets causes repulsion. For fog 'dropletsof given size, the hydrodynamic collision efliciency "increases with increasing radius ofthespray droplet, up to a spray droplet radius of about 1000 microns.

Asa spray droplet of radius R settles, it tends to collect fog droplets whose centres are. at distance less than (R+r) from the centre of the spray droplet. The

.minimum area of influence of the .spray droplet, neglecting any attractive forces .between'spray and fog droplets, is therefore 1r(R+r) the eflfective 'area swept is E1r(R+r). If R -l'- cm., and r=l0- cm., for which E=0.660, then the effective area swept per drop is 0.660 1r (l.1 10" sq. m.=2.s1 1o sq. cm. Hence to sweep effectively. an area of 1 sq. km. (10 sq. cm.) l0 /2.5l 10- =4 10 drops are required. Since each spray drop weighs (4/ 31rR p =42 10'" gm., the weight required is 4X10 4.2 10? gm.= 1.7 10 kg. For a N solution of lauryl dimethyl benzylammonium chloride (equivalent weight 340) this Spraying machines of the type at present used for spreading insecticides over large areas can be used directly or in modified form. Those which employ a hotair blast to assist dissemination of the sprayed material are useful in cases where the spray dropletsmust travel to a considerable height, though with this type of spraying machine, allowance has to be made for evaporation of the spray droplets. An important point is that the spray produced must not contain appreciable proportions by weight of small droplets, which would themselves diminish visibility.

The spray method will disperse fog more rapidly than the smoke method for the following reasons:

(a) A certain time is necessary for the smoke to difiuse through the fog, dissolve in the drops, and come to equilibrium.

(b) The smoke method relies on collisions between i the fog droplets themselves to initiate precipitation. Since these are small, the initial coagulation rate will be low, but the rate increases as larger droplets are a (the reheater), and there mixed with smoke nuclei obtained as described below. The mixture of nuclei and vapour issuing from the reheater is divided into two portions, and drawn through two identical fog chambers formed by lO-litre flasks. When the rate of flow is correctly adjusted, a fog is formed in the flasks. These are interconnected by a separate tube to ensure that conditions of pressure, etc., are the same in each fiask. One of the fogs can then be used as a control, while the other is treated in any desired way.

The smoke nuclei used are silver iodide, calcium chloride or magnesium perchlorate. An acetone solution of silver iodide can be atomised in a spray-type atomiser, using a stream of coal gas, and the issuing mixture of coal gas and solution burnt (using a small flame about 1-2 cm. long). The smoke is passed into the reheater through a glass chimney.

With a given amount of water vapour, a fog which is optically denser (i.e. is of smaller droplet-size) can be obtained by using hygroscopic nuclei (viz. calcium chloride or magnesium perchlorate). There is no great difierence between the fogs obtained with the two types of hygroscopic nuclei.

A fog prepared as above from pure water, especially on (insoluble) silver iodide nuclei is not very stable. There is a tendency to settle out, and small amounts of volatile electrolytes, e.g. ammonia, sulphur dioxide, hydrogen sulphide and hydrogen chloride are added to stabilise the fog. Ammonia is the most effective probably due to the strong adsorption of the hydroxyl ion to give a high negative charge which tends to prevent the growth of the particles by collision. As general practice, the water in the vapour generator .contains small amounts of ammonium hydroxide, and fogs of. various stabilitytimes may be obtained .at will.

Fogs were prepared by the method describedv above in a chamber formed by a vertical tube of height 15 cms. and diameter 4 cms. The smoke nuclei were silver 7 iodide, and the fogs were stabilised by addition of definite amounts of ammonia. The tube was suitably.illu-. minated and the velocity of sedimentation of the fogs under gravity was observed, measurements being made on about fogs (minimum droplet radius about' 10 microns). The velocity of sedimentation of these stabilised fogs was found to be constant during. the time of test (up to 8 seconds), indicating that the drop size was constant. g

Similar tests were made .using identical ,fogs to which a had been added smokes of one of. the cationic-active formed, which settle more rapidly through the fog, collecting other droplets as they go.

With the smoke method, it is necessary to distribute a fairly definite amount of material into the fog, while in the spray method, use of an excess of spray is not detrimental; However, the apparatus. required for the smoke method is less complicated and expensive. With the spray method the transporting of"large droplets to a height of a few hundred feet is difiicult, although sprayingfrom helicopters or' other, aircraft, or from the top of buildings can'help to overcome this problem.

For laboratory tests of themethod of, the invention, fogs may be prepared as followsr A stream of air is drawn through a filter to remove stray nuclei,,and passed as a" stream of fine bubbles through a generator containing ,water maintained at 50-60 C. The resultant stream. of air and water vapour is passed into a chamber maintained at. 80 90 C.

agents cetyl pyridinium chloride, lauryl dimethylbenzylarnmonium chloride, and stearyl dimethyl benzylammonium chloride. For each surface-active agent, measure- 7 ments were made using about 50 fogs, and in each case it was found that the sedimentation velocities increased quite rapidly with time, indicating growth of the particles by collision. The results indicatedthat in a fog of water content about 10 grams per cubic metre, collisions occurred at an average rate of the order of 0.1 collisions per drop per second. The actual value depended on the surface-active agent used, the particle size distribution in the fog, and the amount of smoke added (this being very difficult to control, as theamountsneeded were so small).

For the clearance of e.g. aircraft runways by the spray method, spraying machines are set ,out at suitable distances along the runways. Each spray requires arotating head device and the fields of operation of thesprays are arranged to overlap so as to provide full coverage for .the runway. Clearance is made of-the runway parallel cleared. For example, droplets of radius microns sediment at a rate of 120 ft. per hour, in the absence of convection. Hence if an area one mile square is to be cleared to a minimum height of 100 ft., and the wind velocity is 1 m.p.h., the height of the spray curtain must be 220 ft. It is also desirable to operate some at least of the sprays along the side of the square to prevent sideways drift of fog into the cleared area.

For gradual clearance of larger areas, small smoke generators distributed over the area can be used to lower the charge and hasten precipitation. The rate of emission of the smoke into the area depends on the wind velocity, and the amount must be regulated according to the volume of fog liquid passing through the treated area per unit time, knowing the expected height of penetration of the smoke. a

With either the smoke or spray method, periodical retreatment of the cleared area is generally necessary, as the cleared area generally fills in as fog descends from above by the ordinary sedimentation process (usual rate about 1 cm./sec. in the absence of convection), or as fog diffuses in horizontally. In the presence of wind, the whole cleared area moves downwind, and is replaced by more fog. I

Whilst reference has been made above to a fog it is understood that this term is meant to include aerosols such as mists or clouds which contain droplets of water.

What I claim is:

1. A method of dispersing a fog of moisture droplets comprising forming surface charges on fog dispersing droplets, distributing the latter throughout said fog, and coalescing the moisture droplets'into the interiors of the fog dispersing droplets while maintaining the surface charges on the latter which are thereby efiective to coalesce a large number of said moisture droplets, the fog dispersing droplets being treated with a surface active agent to form said surface charges.

2. A method of dispersing a fog of moisture droplets having electrical surface charges of a particular sign comprising producing a state of electrical surface charge on fog dispersing droplets, which state is non-repellent with respect to surface charges of said particular sign, distributing said fog dispersing droplets throughout said fog, and coalescing the moisture droplets into the interiors of the fog dispersing droplets While maintaining said state of surface charge on the latter which are thereby effective to coalesce a large number of said moisture droplets, the fog dispersing droplets being associated with a surface active agent so as to produce said state of electrical surface charge.

3. A method of dispersing fog according to claim 1,

wherein the surface-active agent is distributed in the fog in the form of a smoke'consisting of finely divided particles of said agent, thereby to facilitate the adsorption of said agent by the fog droplets.

4. A method of dispersing fog according to claim 3, wherein the smoke particles are chosen to have an electrical charge opposite to that of the fog droplets.

5. A method of dispersing fog according to claim 4, wherein, in a given volume of the treated fog, the total electrical charge on the smoke particles is caused to be substantially equal to the total electrical charge on the "fog droplets, whereby the electrical charge on the outside surface of the droplets is substantially neutralized.

6. A method of dispersing fog according to claim 1, wherein the surface-active agent is added to the fog as a dilute solution of said agent in a volatile solvent, whereby, on evaporation of the solvent, the surface-active agent is distributed in the fog in the form of finely-divided particles.

7. A method of dispersing fog according'to claim 6, wherein said solution is distributed in the fog by means of a stream of warm air.

8. A method of dispersing fog according to claim 1,v

wherein the surface-active agent is added to water and distributed in the fog by spraying the water in the form of drops into the fog.

9. A method of dispersing fog according to claim 8, wherein said drops are given an electric charge opposite to that on the fog droplets by the choice of surface-active agent.

10. A method of dispersing fog according to claim 1, wherein the surface-active agent is tetraisoamylammonium chloride.

11. A method of dispersing fog according to claim 1, wherein the surface-active agent is lauryldimethylbenzylammoniumchloride.

12. A method of dispersing fog according to claim 1, wherein the surface-active agent is stearyldimethylbenzylammoniumchloride.

13. A method of dispersing fog according to claim 1,

wherein the surface-active agent is cetylpyridiniumchloride.

References Cited in the file of this patent UNITED STATES PATENTS 1,512,783 Miller Oct. 21, 1924 7 1,928,963 Chaifee Oct. 3,1933 2,052,626 Houghton Sept. 1, 1936 2,160,900 Pleasants June 6, 1939 2,232,728 Pleasants Feb. 25, 1941 2,507,030 Lynch May 9, 1950 OTHER REFERENCES Chemical Reviews, vol. 44, pages 312, 313, and 314.

Cloud Physics, by Perris, page 48.

Rain Making and Other Weather Vagaries, by Humphreys, pages 75, 76 and 77.

On the Local Dissipation of Natural Fog, by Houghton and Radford, pages 16, 17 and 18. 

